The information provided below is not admitted to be prior art to the present invention, but is provided solely to assist the understanding of the reader.
Lithography is the most widely used printing process in which UV and EB inks are employed. Various substrates including paper, carton board, metal, plastic and laminates can be printed by this method. The pros of using UV inks in this process are that they are cleaner and easier to handle. This offsets the fact that they are less versatile in terms of their ink/water balance when running on a press. Printing requirements for offset lithographic inks include high viscosity, good flow and transfer properties, good tack values for sharp imaging, low misting values, insolubility in water but stable emulsion formation with water and sufficient color strength to reproduce the image at a 1–2 micron film thickness. It is desired in the printing industry to produce inks and coatings with high gloss and opacity. In order to convey these desired properties, the base oligomer in a UV/EB curable formulation must provide excellent pigment wetting characteristics to produce an ink of high gloss and opacity or hiding power. Moreover, the base oligomer controls adhesion to the printed substrate. The surface tension, polarity of the ink vehicle, and chemical nature of materials used influence pigment wetting and substrate adhesion characteristics.
Conventional UV lithographic printing requires the use of substantial quantities of a mix of different photoinitiators to ensure optimum cure under all conditions. Moreover, assurance of proper adhesion to low energy substrates, typically used in lithographic printing, also requires the use of a mix of significant quantities of photoinitiator. Traditional photoinitiators (e.g., benzophenone) can be toxic, expensive, and malodorous and contribute to film color, which can limit applicability in white and light-colored inks.
The amount of external photoinitiator in lithographic ink formulations can be significantly reduced by using the acrylate oligomer technology described in patents U.S. Pat. No. 5,945,489 and 6,025,410 (both Ashland, Inc.). These patents disclose uncrosslinked resins prepared via the Michael addition reaction of β-dicarbonyl compounds with multifunctional acrylates. The invention disclosed here demonstrates the advantageous use of these uncrosslinked resins alone or modified by reaction/blending with additional materials in ink formulations for lithographic printing applications. These additional materials include a variety of acrylic monomers and oligomers, free-radically reactive vinyl monomers, primary and secondary and tertiary amines, acid-functional materials, siloxanes, elastomers, waxes, etc. to modify and improve printing performance, and pigments to impart color to inks.
Lithographic-printing inks based on the resins described above can be cured by all methods typically used to crosslink acrylic materials. Cure, or crosslinking, is usually accomplished through a free radical chain mechanism, which may require any of a number of free radical-generating species such as peroxides, hydroperoxides, REDOX combinations, etc., which decompose to form radicals when heated; additionally, curing can be achieved at ambient temperature in the presence of amines or transition metal promoters. Ultraviolet (UV) radiation is another means of initiating reaction by decomposing an appropriate photoinitiator to form free radicals. Electron beam (EB) radiation can also be used to effect cure.
The lithographic inks based on this new self-initiating resin technology can UV-cure at commercial line speeds to give prints with good color strength at significantly lower photoinitiator levels. The behavior of these inks on a hydroscope was very similar to that of commercial inks with good viscosity, tack and torque values.
The novel ink formulations disclosed here exhibit performance properties that make them very effective when printed on a range of substrates. These properties can be modified greatly depending upon oligomer composition and coating formulation rather than by blending with additives, as is done in traditional UV ink systems. The inks can exhibit wide ranges of gloss, rheological performance, flexibility, stain resistance, scratch resistance, weather resistance, solvent resistance, etc. Almost any desired ink performance parameter can be attained by proper selection of the raw material building blocks used to make the oligomers that form the basis of the ink formulation.
Other objects and advantages will become apparent from the following disclosure.